1. Field of the Invention
The present invention relates to an antenna and a wireless IC device, and more particularly, to an antenna for a wireless IC device used in an RFID (Radio Frequency Identification) system and a wireless IC device including the antenna.
2. Description of the Related Art
In general, RFID systems in which a reader/writer that generates an electromagnetic wave and a wireless IC (referred to as an IC tag, a wireless IC device, etc.) that is attached on an article, a container or other suitable object and stores predetermined information communicate with each other in a non-contact manner to transfer information have been developed as a management system for articles. Communication with the reader/writer is achieved by coupling the wireless IC with an antenna (emission plate).
As a wireless IC of this type, Japanese Unexamined Patent Application Publication No. 10-293828 describes a wireless IC in which a coil L11 of an ID module 100 and a coil L12 of an a coil module 110 are coupled and set to resonate at a predetermined frequency, as shown in FIG. 14. The ID module 100 causes the primary coil L11 and a capacitor C11 to produce parallel resonance. The coil module 110 is configured to have a closed loop by electrically connecting the ends of two coils L12 and L13 to each other. Energy is transferred between the two coils L12 and L13 by a current. That is, a magnetic field is emitted when the current, which is generated by the coil L12 coupled with the primary coil L11, passes through the coil L13.
However, increasing the inductance value of the coil for emission L13 and decreasing the inductance value of the coupling coil L12 in order to increase the magnetic field energy to be received from the reader/writer causes the voltage at the ends of the coil L12 to decrease. Therefore, there is a problem in that the amount of current flowing between the two coils L12 and L13 is decreased and sufficient energy is not transferred from the IC to the coil for emission L13, which reduces the communication distance. In addition, when the emission coil L13 is close to the antenna of the reader/writer or is close to another wireless IC, the inductance value varies due to the mutual inductance and the resonant frequency of the primary coil L11 also varies. As a result, reading by the reader/writer may be disabled.